This invention relates to a heating apparatus and more particularly to a heating apparatus especially for use with food warming trays.
Food warming apparatus has been found to be very useful in many situations. For instance when used as part of food trays in a hospital it maintains the food at an optimum temperature even though delivery and actual eating of the food take substantial time.
It was common in the prior art to employ a standard resistance heater coupled with a thermostat to regulate the heat generated in the food warming apparatus. There were many disadvantages with this warming apparatus which are overcome by using a positive temperature coefficient of resistance material along with a heat transfer plate to provide the heat needed. A food warming apparatus of this type is described in U.S. Pat. No. 3,720,807 issued Mar. 13, 1973 to the assignee of the instant invention. Even though great improvement over prior food warming apparatus is achieved using the above-mentioned PTC heater apparatus occasional sealing and low resistance contacting problems occur. These food warming devices are typically a part of trays which are immersed in water and therefore any leakage causes problems. Additionally proper low resistance contacting with the PTC pill is needed for long, reliable operation.
It is an object of the present invention to provide a food warming apparatus with an improved low resistance contacting and sealing system. Another object of this invention is to provide an apparatus which is easily produced and assembled at minimal cost. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly the present invention comprises a food warming apparatus employing at least one PTC resistance heater pill in thermal communication with a relatively large heat transfer plate with a flange having high heat conductivity. Heat generated in the pill is conducted away from the pill to the plate thus keeping the pill in a low resistance state until the plate heats up at which time the pill changes to a high resistance state for reducing heat output. The pill is metallized on opposite faces and secured to the heat transfer plate. A base member having a recess or trough for receiving and positioning the flange of the plate and having a spring secured thereto by a post to make electrical contact with the PTC pill is made out of electrically and thermally insulative material. A sealing silicon rubber compound is disposed in the trough to provide a water tight seal totally surrounding the flange and to retain the heat transfer plate in position to provide a low resistance electrical contact between the PTC and the spring by properly aligning and permanently positioning the spring in contact with one of the metallized surfaces of the pill. An electrical circuit is made from the heat transfer plate through the PTC pill and spring to an electrical lead connected to the spring.